Just three years ago, Apple wasn't in the mobile-phone business at all. Since then, its game-changing iPhone has become the most influential smartphone in the world. Now, on June 24, the company will roll out the fourth generation of the device, called the iPhone 4.

While attractive, capable new smartphones emerge regularly from competitors, a new iPhone deserves special attention for two reasons. First, the device lies at the center of a huge ecosystem of 225,000 apps, plus popular related gadgets like Apple's iPod Touch connected media player and iPad tablet, which collectively are approaching 100 million units sold. Second, the iPhone's multitouch, gesture-based interface; elegant Web browser; sophisticated music and video playback; and other features have been emulated on many competing devices, so what Apple does affects the whole industry.

I've been testing the iPhone 4 for more than a week. In both hardware and software, it is a major leap over its already-excellent predecessor, the iPhone 3GS.